“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Sunday, September 12, 2010

More on Moller International and the "Skycar"

Back in March of 2009, I made a couple of posts (here and here) about the Moller Skycar. I questioned some of the claims made for the vehicle with respect to fuel economy and speed. The other day, a comment on the second of my posts related to Moller mentioned that my posts had been discussed on the Moller blog site. I'll quote the comment by a blog visitor and the answer by Bruce Calkins, Moller International's General Manager, in full:

A long time skeptic  - Query re: concerns about range and fuel efficiency
I was curious to know what Moller International thinks of the speculations by this blogger here: http://hamiltonianfunction.blogspot.com/2009/03/moller-sky-car.html and here: http://hamiltonianfunction.blogspot.com/2009/03/horsepower-fuel-efficiency-and_31.html I'm not sure I agree with all his assumptions, but he makes a pretty damning case. I would be interested to here the company's reaction.

Bruce Calkins  - Skeptics talk about fuel economy of the Skycar
There is a saying that a little knowledge is a dangerous thing. The blog entries seem to fit into this category. While the maximum hp and burn rate are correct, the assumption that we remain at that rate for anything more than the few short seconds while we are operating in VTOL mode is not. Our hp requirement in cruise is about 120 hp, with the corresponding fuel burn rate. Range projections are based on the reduced fuel burn rate. We are currently evaluating a hybrid fuel-electric propulsion system that might reduce the total installed power requirement replacing it with a temporary electrical "boost" system for the VTOL-mode operations. If you would like a more detailed paper on the subject, email me.
 Before I comment on this I want to state that Dr. Moller is, without a doubt, a brilliant man and an extraordinary aerodynamic engineer. Certainly, he's far beyond my expertise. That said, he and those with whom he works have a long history of making claims that are never fulfilled. A good place to start reading about this aspect is at Wikipedia's Skycar page.

Now, Mr. Calkins is stating that the Skycar will cruise using 120 horsepower. The Skycar specifications page states that the cruise speed at 25,000 feet is 305 m.p.h. and that "max mileage" is greater than 20 m.p.g. It's not stated what airspeed is utilized to achieve this mileage. It may be very much lower than 305 m.p.h. In my airplane, the maximum range airspeed (that is, the airspeed at which I can achieve the greatest number of miles per gallon) is about 24% lower than normal cruise speed. Interestingly, on another page the claim is that "M400 Skycar can cruise comfortably at 275 MPH (maximum speed of 375 MPH) and achieve up to 20 miles per gallon on clean burning, ethanol fuel." I'll use this and supplement it with information in a paper hosted on Moller's web site. However, one thing I note is that none of the referenced pages give a categorical statement that the Skycar can achieve a cruise speed of, say, 275 miles per hour while it's getting 20 m.p.g. Instead, statements such as that quoted above can be backed away from by saying "well, yes, it can get 20 m.p.g. at the minimum point on the thrust required as function of airspeed curve," typically not far above the maximum L/D (lift/drag) ratio airspeed.

But let's proceed. We'll start with the assumption of steady flight in cruise. The conservative claim above is that the Skycar will do 275 m.p.h. in cruise and Calkins states that it uses "about 120 horsepower." OK, as I've mentioned any number of times, power is equal to force times speed, so force is equal to power divided by speed. The Google calculator makes this division and  handily converts the units to newtons, yielding a thrust of 728 newtons.

Now, as we learn in high school physics or in airman ground school, for an airplane in unaccelerated flight (as the Skycar would be under the cruise condition assumed here) thrust=drag, and lift=weight. Hence, we're considering a total (lift induced plus parasitic) drag of 728 newtons for an aircraft travelling at 275 miles per hour. We need to determine if this is plausible. Without a lot of mathematical derivation, it's pretty easy to show that, in unaccelerated cruise flight, Thrust=Weight/(Lift/Drag). That means that Lift/Drag=Weight/thrust. Here we have (using 2400 pounds or 10,676 newtons as the "gross weight") that Lift/Drag=10,676/728=14.66. But the specifications page referenced above has the so-called maximum L/D (the maximum ratio of lift to drag) as 12.5. Now, this maximum ratio is at a specific airspeed, any other speed will produce a lower L/D ratio. 14.7 cannot be achieved.

The Moller International web site does not give sufficient data to go much further (wing span, wing area, flate plate area, etc.) so I can't really do much more than this. I will say that the figures claimed by Moller International are likely not outlandish but, in my opinion, they are very significantly exaggerated.

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